N. ^UK et al.: PROPERTIES OF PARTICLEBOARDS MADE BY USING AN ADHESIVE ...
PROPERTIES OF PARTICLEBOARDS MADE BY USING AN ADHESIVE WITH ADDED LIQUEFIED WOOD
LASTNOSTI IVERNIH PLO[^, IZDELANIH Z UPORABO LEPILA Z DODANIM UTEKO^INJENIM LESOM
Nata{a ^uk1,2, Matja` Kunaver1,2, Sergej Medved3
1National Institute of Chemistry, Hajdrihova 19, SI-1000 Ljubljana, Slovenia
2Center of Excellence for Polymer Materials and Technologies, Tehnolo{ki Park 24, SI-1000 Ljubljana, Slovenia 3Biotechnical Faculty, Department of Wood Science and Technology, Ro`na dolina, Cesta VIII/34, 1000 Ljubljana, Slovenia
natasa.cuk@ki.si
Prejem rokopisa – received: 2011-02-10; sprejem za objavo – accepted for publication: 2011-03-28
In this study, three-layer particleboards were produced using liquefied wood in an adhesive mixture. The influence of two different formaldehyde resins: melamine-formaldehyde and melamine-urea-formaldehyde resin and two various catalysts:
ammonium sulfate and ammonium formate on the particleboard properties was investigated. There were also two pressing parameters examined: temperature and time. The following physical and mechanical properties of particleboards were measured: board thickness, density, moisture content, bending strength and modulus of elasticity, internal bonding strength, surface soundness, thickness swelling and formaldehyde content. The results showed that properties of produced particleboards were better when melamine-formaldehyde resin and ammonium formate as a catalyst were used in combination with liquefied wood in the adhesive mixture. Also, mechanical properties were improved as the press time and press temperature increased.
The optimal mechanical properties of particleboards made with the utilization of the liquefied wood in the adhesive mixture were achieved at 3 min press time and 180 °C press temperature using melamine-formaldehyde resin and 3 % of ammonium formate as a catalyst. A very important characteristic is the low formaldehyde content and that is extremely important in the provision of better quality of life. With the liquefaction of wood and the application of liquefied wood in the particleboard production wood biomass exploitation could be significantly increased. We are able to produce new materials from renewable resources that can be a great alternative to the raw materials, originating from crude oil.
Keywords: liquefied wood, renewable resources, particleboards, mechanical properties
Izdelali smo trislojne iverne plo{~e, kjer smo za pripravo lepila uporabili uteko~injen les. Ugotavljali smo, kako v kombinaciji z uteko~injenim lesom na lastnosti ivernih plo{~ vplivata dve razli~ni formaldehidni smoli: melamin-formaldehidna in melamin-urea-formaldehidna smola, ter dva razli~na katalizatorja: amonijev sulfat in amonijev formiat. Ugotavljali smo tudi vpliv dveh parametrov stiskanja, in sicer temperature stiskanja in ~asa stiskanja. Fizikalne in mehanske lastnosti, ki smo jih ivernim plo{~am dolo~ili, so bile: debelina plo{~e, gostota, vsebnost vlage, upogibna trdnost in modul elasti~nosti, razslojna trdnost, ~vrstost povr{ine, debelinski nabrek in vsebnost prostega formaldehida. Rezultati so pokazali, da smo izbolj{ali lastnosti ivernih plo{~, ko smo v kombinaciji z uteko~injenim lesom uporabili melamin-formaldehidno smolo in amonijev formiat kot katalizator. Ugotovili smo tudi, da so se lastnosti izdelanih ivernih plo{~ s ~asom in temperaturo stiskanja izbolj{ale. Optimalne lastnosti ivernih plo{~, pri katerih smo lepilni me{anici dodali uteko~injen les, smo dosegli pri ~asu stiskanja 3 min in temperaturi stiskanja 180 °C, pri uporabi melamin-formaldehidne smole in 3 % amonijevega formiata kot katalizatorja. Zelo pomembna pri izdelanih ivernih plo{~ah je tudi majhna vsebnost prostega formaldehida, kar mo~no vpliva na izbolj{anje kvalitete bivanja. Z uteko~injanjem lesa in njegovo uporabo pri izdelavi ivernih plo{~ pove~amo izrabo lesne biomase. Iz obnovljivih virov tako lahko sintetiziramo nove materiale, ki so odli~na alternativa materialom, ki jih sicer pridobivamo iz surove nafte.
Klju~ne besede: uteko~injen les, obnovljivi viri, iverne plo{~e, mehanske lastnosti
1 INTRODUCTION
Wood is recyclable, renewable, biodegradable and one of the most abundant natural polymers. It can be used for many different purposes and converted to many useful industrial chemicals. Nowadays, it can also be liquefied using different polyhydric alcohols and acid catalysts. Celullose, hemicelullose and lignin are the main wood components and during liquefaction these components are decomposed – depolymerized. The final product has lots of free hydroxyl groups and can therefore be used to prepare an adhesive for gluing the wood and wood composites. Particleboards for which we use 10–15 % of adhesive (based on the weight of particles) represent almost 60 % of the wood composites production.
To produce particleboards wood is broken down into particles of various size and glued together. Wood par- ticles are bonded using synthetic adhesives and pressed into boards at high temperature and high pressure. The pressing operation provides increased density and strength.1
The properties of particleboards are engineered in a manner to meet many service requirements. Particle- boards can be made in large panel sizes, with a full range of thicknesses. With the introduction of continuous presses, endless boards emerge and are subsequently cut into any length desired.2There are many factors affecting the properties of the particleboards and the most important are:
• species of wood, fiber structure, density, hardness, compressibility;
Original scientific article/Izvirni znanstveni ~lanek MTAEC9, 45(3)241(2011)
• form and size of raw wood;
• share of bark;
• non-wood lignocellulose materials;
• type and size of particles;
• method of particle drying;
• particle screening and separating, particle size distri- bution;
• type and amount of binding agents;
• method of mat formation, structure of particleboard;
• moistening of particles prior to pressing, final moi- sture content of board, conditioning;
• curing conditions;
• thickness of board;
• sand content of particleboard;
• surface quality;
• primings, coats of varnish or lacquer;
• laminating, veneering, overlaying.3
Over the years many different lignocellulosic mate- rials beside wood were used in particleboard production:
coconut chips,4 paper sludge,5 waste tea leaves,6 castor stalks,7 wheat straw,8 flax shiv,9 kenaf stalks,10 needle litter,11waste grass clippings,12bagasse,13saline creeping wild rye,14peanut hull,15cotton carpel,16vine prunings,17 kiwi prunings,18 waste tissue paper and corn peel,19 almond shell20and others.
Next to the commonly used urea-formaldehyde resin for particleboard manufacturing, there were also other adhesives applied: melamine-modified urea-formalde- hyde resin,21 phenol-formaldehyde resin and melamine- urea-phenol-formaldehyde resin,22 PTP resin,23 soy-pro- tein adhesive,8, 24 cement,25 rice bran adhesive,26 EMDI isocyanate resin,4MDI resin.8
Lee and Liu prepared resol resin based on liquefied bark and used it for particleboard production.27Particle- board made from liquefied Taiwan acacia bark, using H2SO4as a liquefaction catalyst, had the best properties in bending strength, internal bonding strength and thickness swelling. Liquefied wood was also used in preparation of phenol-formaldehyde resin,28 polyure- thane resin,29isocyanate adhesive30and epoxy resin.31
Since we have already proved in our previous research that liquefied wood can be used in the adhesive mixture for particleboard production,32 the aim of this research was to evaluate the influence of some factors on the properties of particleboards made from liquefied wood. We investigated the impact of two different formaldehyde resins and two various catalysts used in combination with liquefied wood in the adhesive mixture on the properties of particleboards. We also examined the influence of two pressing conditions: temperature and time. Other parameters such as liquified wood loading, resin content and press pressure were held constant.
2 EXPERIMENTAL 2.1 Wood liquefaction
Sawdust of spruce wood (Picea abies (L.) Karst.) was liquefied using the mixture of glycerol and diethylene glycol (mass ratio 80/20,) as a liquefying reagent and p-toluenesulphonic acid as a catalyst. The mass ratio of wood sawdust to glycols was 1 : 3 and the amount of p-toluenesulphonic acid was 3 % based on the mass of glycols.
The mixture of glycols and acid was charged into three neck glass reactor equipped with mechanical stirrer, thermometer, condenser and external heating.
When the mixture in the reactor reached temperature 160
°C wood sawdust was added. After all the wood sawdust was added the temperature was elevated to 180 °C and maintained for 90 min to carry out the liquefaction reaction. After the liquefaction the liquefied product was cooled down.
2.2 Adhesive preparation
The adhesive was prepared with the addition of liquefied wood (LW) to the melamine-formaldehyde (MF) or the melamine-urea-formaldehyde (MUF) resin.
Both resins, MF and MUF, were received from Melamin Ko~evje where they also measured the properties. The properties of MF and MUF resin are presented inTable 1. The liquefied wood loading was 30 % based on the mass of adhesive needed (dry content based), other 70 % was either MF or MUF resin. The mixture was thoroughly blended, then water and catalyst were added.
The catalyst was only added into the adhesive mixture for the core layer. Catalysts used were 20 % solution of ammonium sulfate ((NH4)2SO4) and 10 % solution of ammonium formate (NH4HCO2). The catalyst content was 3 % based on the weight of the adhesive mixture, except when the influence of catalyst on the properties of particleboards was investigated. The whole resin mixture was blended for a few minutes before use.
Table 1:Characteristics of MF and MUF resin Preglednica 1:Karakteristike MF in MUF smole
Properties MF MUF
Solid content (58 ± 1) % (63 ± 2) %
Viscosity* 18–22 s 80–200 s
Free formaldehyde content below 0.2 % max. 0.5 %
pH value 9–10 9.2–9.5
Stability (at 20 °C) 20 d 2 months
*viscosity was determined according to the standard DIN EN ISO 2431,f= 4 mm, 20 °C
2.3 Particleboard production
Three-layer particleboards of 500 mm by 500 mm in size, 16 mm of thickness and 0.600 g/cm3 of target density were produced in laboratory conditions. The
resin content was 10.50 % in the surface layer and 6.50
% in the core layer. The resin content was calculated based on the dry weight of wood particles.
Wood particles mixture composed from 75 % of spruce wood (Picea abies(L.) Karst.) and 25 % of beech wood (Fagus sylvatica L.) was first dried at 70 °C to obtain the moisture content approximately 4 %, then the resin was added. After blending for several minutes the glued wood particles were hand-formed into a mat using wood mold. For the determination of resin and catalyst type impact on the particleboard properties, the wood mat was hot-pressed for 3 min at temperature 180 °C.
Later the press times were (2, 2.5, 3 and 3.5) min at the press temperature 180 °C and the press temperatures were 140 °C, 160 °C, 180 °C and 200 °C at the press time of 3 min. Before testing, all particleboards were conditioned for 4–5 d at temperature 20 °C and relative humidity 65 %.
2.4 Particleboard testing
The physical and mechanical properties of produced particleboards were tested according to the European standards: the board thickness EN 324, the density EN 323, the moisture content EN 322, the bending strength and the modulus of elasticity EN 310, the internal bonding strength EN 319, the surface soundness EN 311, the thickness swelling EN 317 and the formaldehyde content EN 120.
3 RESULTS AND DISCUSSION
The results of the physical and mechanical properties of laboratory made particleboards bonded with LW combined with the MF and the MUF resin are presented inTable 2. The results indicate that mechanical proper- ties of particleboards were up to 25 % better when MF resin was used. The reason may be that the resin con- taining more melamin is more reactive. Also, the cyclic structure of the melamine imparts greater stability to the resulting linkages. The three amine groups assure a three-dimensional, cross-linked molecular structure when fully cured.33Consequently, mechanical properties of particleboards using LW and MF resin were better in comparison to the particleboards where LW and mela- mine enhanced UF (MUF) resin was used. Nevertheless, in both cases, the properties satisfied the standard requirements. There was no major difference in thick- ness swelling with regard to the resin type used.
From previous research we have already found out that the addition of LW in the adhesive mixture reduces the content of formaldehyde in the boards.32 In this research we have again obtained low formaldehyde content. We observed that formaldehyde content was lower when MF resin was used. Lower formaldehyde content at use of MF resin, compared to formaldehyde content at use of MUF is related to the nature of reaction between formaldehyde and melamine or formaldehyde
and melamine-urea, as it was reported by Eom et al.34 Such boards can easily be used in dry conditions for general and interior purposes.
Table 2:Properties of particleboards produced by using an adhesive mixture made of LW combined with MF or MUF resin
Preglednica 2: Lastnosti ivernih plo{~, izdelanih iz uteko~injenega lesa v kombinaciji z MF in MUF smolo v lepilni me{anici
Particleboard properties LW-MF LW-MUF
Board thickness (mm) 15.71 16.18
Density (g/cm3) 0.627 0.610
Moisture content (%) 7.98 7.70
Bending strength (N/mm2) 13.42 11.06 Modulus of elasticity (N/mm2) 2322 1823 Internal bonding strength (N/mm2) 0.47 0.45 Surface soundness (N/mm2) 1.28 1.18 Thickness swelling (%) 19.51 19.97 Formaldehyde content
(mg/100 g of dry board) 1.40 2.55
Table 3:Properties of particleboards produced by using an adhesive with added LW and ammonium formate or ammonium sulfate as a catalyst
Preglednica 3: Lastnosti ivernih plo{~, izdelanih iz uteko~injenega lesa, dodanega lepilni me{anici pri uporabi amonijevega formiata in amonijevega sulfata kot katalizatorja
Particleboard
properties NH4HCO2 (NH4)2SO4 Catalyst content 1 % 2 % 3 % 4 % 1 % 2 % 3 % 4 % Board thickness
(mm) 15.58 15.70 15.71 15.69 16.04 15.83 15.53 15.71 Density (g/cm3) 0.658 0.643 0.627 0.660 0.637 0.642 0.659 0.651 Moisture content
(%) 7.80 7.55 7.98 8.27 8.23 7.49 7.18 7.08 Bending strength
(N/mm2) 12.37 12.71 13.42 12.70 10.48 11.96 12.98 11.71 Modulus of
elasticity (N/mm2) 2286 2327 2322 2240 2091 2193 2404 2321 Internal bonding
strength (N/mm2) 0.40 0.54 0.47 0.47 0.34 0.40 0.46 0.39 Surface soundness
(N/mm2) 1.22 1.22 1.28 1.18 1.01 1.09 1.14 1.24 Thickness swelling
(%) 20.82 18.77 19.51 18.20 21.62 20.74 21.33 22.00 Formaldehyde
content (mg/100 g of dry board)
2.51 2.06 1.40 1.22 3.44 2.85 2.71 2.25
We also investigated the impact of type and amount of catalyst on the particleboard properties. We used two different ammonium salts: ammonium formate (NH4HCO2) and ammonium sulfate ((NH4)2SO4). The results of particleboard testing are shown in Table 3. It can be seen that the properties of particleboards were better when ammonium formate was used. We also varied the quantity of the catalyst from 1 % to 4 %. The properties of particleboards were the best when the 3 % loading of each catalyst was applied. When comparing the efficiency of both catalysts and the influence of the amount of the catalyst the difference in thickness swelling is not very distinctive. Nevertheless, there are some differences in formaldehyde content. With higher amount of catalyst the formaldehyde content was lower
for the both catalysts. Furthermore, the formaldehyde content was higher when ammonium sulfate was used.
Further on, we evaluated the influence of pressing conditions, i.e. time and temperature, on the properties of particleboards made by using an adhesive with added liquefied wood. The results of testing particleboards pressed at different times are presented in Table 4. The prolongation of the press time resulted in an improve- ment of mechanical properties. The best properties were achieved at press time of 3 min. The time of pressing can be shorten to 2.5 min and consequently, the energy could be saved significantly. Namely, the reduction of press time means greater level of production, i.e. increased output, and consequently, the reduction of the unit production costs. Of course, press time reduction must be carefully watched because of the impact on the board quality. The press time had no larger effect on the thickness swelling and the formaldehyde content.
Table 4:Properties of particleboards produced by using an adhesive with added LW pressed at different times
Preglednica 4: Lastnosti ivernih plo{~, izdelanih iz uteko~injenega lesa, dodanega lepilni me{anici pri razli~nih ~asih stiskanja
Particleboard properties 2 min 2,5 min 3 min 3,5 min Board thickness (mm) 16.35 16.15 16.12 15.85 Density (g/cm3) 0.614 0.595 0.596 0.624 Moisture content (%) 6.18 5.97 5.43 5.53 Bending strength (N/mm2) 8.53 9.53 10.62 9.65 Modulus of elasticity
(N/mm2) 1872 1806 2106 1904
Internal bonding strength
(N/mm2) 0.37 0.52 0.46 0.48
Surface soundness (N/mm2) 0.95 0.94 1.03 0.93 Thickness swelling (%) 19.58 19.59 19.75 17.82 Formaldehyde content
(mg/100 g of dry board) 3.49 2.94 2.83 3.02
Table 5:Properties of particleboards produced by using an adhesive with added LW pressed at different temperatures
Preglednica 5: Lastnosti ivernih plo{~, izdelanih iz uteko~injenega lesa, dodanega lepilni me{anici pri razli~nih temperaturah stiskanja
Particleboard properties 140 °C 160 °C 180 °C 200 °C Board thickness (mm) 17.51 16.47 16.12 16.27 Density (g/cm3) 0.592 0.629 0.596 0.628 Moisture content (%) 6.03 5.63 5.43 4.77 Bending strength (N/mm2) 8.10 9.34 10.62 10.99 Modulus of elasticity
(N/mm2) 1655 1974 2106 2037
Internal bonding strength
(N/mm2) 0.26 0.44 0.46 0.43
Surface soundness (N/mm2) 0.94 0.99 1.03 0.96 Thickness swelling (%) 19.86 20.56 19.75 20.44 Formaldehyde content
(mg/100 g of dry board) 2.74 2.76 2.83 2.71
The results of testing of particleboards which were pressed at various temperatures are presented inTable 5.
When the press temperature was elevated, the properties of particleboards were improved. The optimal properties were obtained at press temperature of 180 °C, while the
temperature of 140 °C was too low to achive required strength and the temperature of 200 °C did not provide better quality of the board than of the 180 °C. The press temperature can be lowered to 160 °C and the properties would still satisfy the European standards requirements.
The press temperature did not have much effect on both the thickness swelling and the formaldehyde content, what was also observed when evaluating the influence of the press time.
4 CONCLUSIONS
The three-layer particleboards were successfully produced using adhesive with the addition of 30 % of liquefied wood. The physical and mechanical properties of produced particleboards met the European standard requirements and were better when using melamine- formaldehyde resin and 3 % of ammonium formate. The optimal properties of particleboards were achieved at the press time of 3 min and the press temperature 180°C. By shortening the press time to 2.5 min or by decreasing the press temperature to 160°C reduction of the energy costs could be achieved. Nevertheless, the additional research will be needed to confirm these findings. Particleboards made with the adhesive with added liquefied wood have low formaldehyde content, and that is of special import- ance when using particleboards for interior applications.
With the liquefaction of wood and application of liquefied wood in particleboard production the wood biomass explotation can be increased. That would enable efficient lignocellulosic waste recycling, increase renewable resources usage and reduce the crude oil consumption.
ACKNOWLEDGEMENTS
The authors wish to thank for the support to Ministry of Higher Education, Science and Technology of the Republic of Slovenia within the contract No.
3211-10-000057 (Center of Excellence for Polymer Materials and Technologies), GG Postojna for their financial and material support, Melamin Ko~evje for their material support and Janez Renko for his technical help.
5 REFERENCES
1Rowell R. M., Handbook of wood chemistry and wood composites, CRC Press, Boca Raton 2005, 284, 293
2Moslemi A. A., Particleboard, Volume 1: Materials, Southern Illinois University Press Carbondale and Edwardsville 1974, 6
3Kollmann F. F. P, Kuenzi E. W., Stamm A. J., Principles of wood science and technology, Volume II: Wood based materials, Springer- Verlag, Berlin, Heidelberg, New York 1975, 463
4Papadopoulos A. N., Traboulay E. A., Hill C. A. S., One layer experimental particleboard from coconut chips – (Cocos nuciferaL.), Holz als Roh- und Werkstoff, 60 (2002), 394–396
5Taramian A., Doosthoseini K., Mirshokraii S.A., Faezipour M., Particleboard manufacturing: an innovative way to recycle paper sludge, Waste Management, 27 (2007), 1739–1746
6Yalinkilic M. K., Imamura Y., Takahashi M., Kalaycioglu H., Nemli G., Demirci Z., Ozdemir T., Biological, physical and mechanical properties of particleboard manufactured from waste tea leaves, International Biodeterioration & Biodegradation, 41 (1998), 75–84
7Grigoriou A. H., Ntalos G. A., The potential use of Ricinus Communis L. (Castor) stalks as a lignocellulosic resource for particleboards, Industrial Crops and Products, 13 (2001), 209–218
8Mo X., Cheng E., Wang D., Sun X. S., Physical properties of medium-density wheat straw particleboard using different adhesives, Industrial Crops and Products, 18 (2003), 47–53
9Papadopoulos A.N., Hague J.R.B., The potential for using flax (Linum usitatissimumL.) shiv as a lignocellulosic raw material for particleboard, Industrial Crops and Products, 17 (2003), 143–147
10Kalaycioglu H., Nemli G., Producing composite particleboard from kenaf (Hibiscus cannabinusL.) stalks, Industrial Crops and Products, 24 (2006), 177–180
11Nemli G., Yildiz S., Gezer E. D., The potential for using the needle litter of Scotch pine (Pinus sylvestris L.) as a raw material for particleboard manufacturing, Bioresource Technology, 99 (2008), 6054–6058
12Nemli G., Demirel S., Gümüskaya E., Aslan M., Acar C., Feasibility of incorporating waste grass clippings (Lolium perenneL.) in par- ticleboard composites, Waste Management, 29 (2009), 1129–1131
13Xu X., Yao F., Wu Q., Zhou D., The influence of wax-sizing on dimension stability and mechanical properties of bagasse particle- board, Industrial Crops and Products, 29 (2009), 80–85
14Li B., Zheng Y., Pan Z., Hartsough B., Improved properties of medium-density particleboard manufactured from saline creeping wild rye and HDPE plastic, Industrial Crops and Products, 30 (2009), 65–71
15Guler C., Copur Y., Tascioglu C., The manufacture of particleboard using mixture of peanut hull (Arachis hypoqaea L.) and European Black pine (Pinus nigra Arnold) wood chips, Bioresource Techno- logy, 99 (2008), 2893–2897
16Alma M. H., Kalaycioglu H., Bektas I., Tutus A., Properties of cotton carpel-based particleboards, Industrial Crops and Products, 22 (2005), 141–149
17Ntalos G. A., Grigoriou A. H., Characterization and utilisation of vine prunings as a wood substitute for particleboard production, Industrial Crops and Products, 16 (2002), 59–68
18Nemli G., Kirci H., Serdar B., Ay N., Suitability of kiwi (Actinidia sinensis Planch.) prunings for particleboard manufacturing, Industrial Crops and Products, 17 (2003), 39–46
19Lertsutthiwong P., Khunthon S., Siralertmukul K., Noomun K., Chandrkrachang S., New insulating particleboards prepared from
mixture of solid wastes from tissues paper manufacturing and corn peel, Bioresource Technology, 99 (2008), 4841–4845
20Gürü M., Tekeli S., Bilici I., Manufacturing of urea-formaldehyde- based composite particleboard from almond shell, Materials &
Design, 27 (2006), 1148–1151
21Hse C.-Y., Fu F., Pan H., Melamine-modified urea-formaldehyde resin for bonding particleboard. Forest Products Journal, 58 (2008), 56–61
22Wagner D., Schwarzinger C., Leidl M., Schmidt H., Endesfelder A., Particleboards from acetylated wood flakes, Monatshefte für Chemie, 138 (2007), 321–325
23Boquillon N., Elbez G., Schönfeld U., Properties of wheat straw particleboards bonded with different types of resin, Journal of Wood Science, 50 (2004), 230–235
24Wang D., Sun X. S., Low density particleboard from wheat straw and corn pith, Industrial Crops and Products, 15 (2002), 43–50
25Erakhrumen A. A., Areghan S. E., Ogunleye M. B., Larinde S. L., Odeyale O. O., Selected physico-mechanical properties of cement- bonded particleboard made from pine (Pinus caribaea M.) sawdust-coir (Cocos nuciferaL.) mixture, Scientific Research and Essay, 3 (2008), 197–203
26Pan Z., Cathcart A., Wang D., Properties of particleboard bond with rice bran and polymeric methylene diphenyl diisocyanate adhesives, Industrial Crops and Products, 23 (2006), 40–45
27Lee W.-J., Liu C.-T., Preparation of liquefied bark-based resol resin and its application to particleboard, Journal of Applied Polymer Science, 87 (2003), 1837–1841
28Lee W.-J., Chen Y.-C., Novolak PF resins prepared from phenol liquefiedCryptomeria japonicaand used in manufacturing moldings, Bioresource Technology, 99 (2008), 7247–7254
29Wei Y., Cheng F., Li H., Yu J., Synthesis and properties of poly- urethane resins based on liquefied wood, Journal of Applied Polymer Science, 92 (2004), 351–356
30Tohmura S.-I., Li G.-Y., Qin T.-F., Preparation and characterization of wood polyalcohol-based isocyanate adhesives, Journal of Applied Polymer Science, 98 (2005), 791–795
31Kishi H., Fujita A., Miyazaki H., Matsuda S., Murakami A., Synthesis of wood-based epoxy resins and their mechanical and adhesive properties, Journal of Applied Polymer Science, 102 (2006), 2285–2292
32Kunaver M., Medved S., ^uk N., JasiukaitytëE., Poljan{ek I., Strnad T., Application of liquefied wood as a new particle board adhesive system, Bioresource Technology, 101 (2010), 1361–1368
33Marra A. M., Technology of wood bonding, Principles in Practice, Van Nostrand Reinhold, New York 1992, 89
34Eom Y. G., Kim, S., Baek I-C., Kim, H-J. Comparison of Formal- dehyde Emission of Wood-Based Panels With Different Adhesive- hardener Combinations by Gas Chromatography and Standard Methods, Mokchae Konghak, 33 (2005), 29–39
